(1) Field of the Invention
The present invention relates to an optical modulator which modulates light from a semiconductor laser or light source at high speeds for transmission, and more particularly, to such an optical module for use in high-speed, large-capacity optical communications and optical interconnections.
(2) Description of the Related Art
Optical communication networks have enabled communication of large quantities of information. However, according to the estimates of scholars, the communication infrastructure in Japan will experience shortages of capacity due to rapid increases in the amount of data communications and will become incapable of responding to the demand within five years, at the earliest. It is said that the tendency for capacity shortage will be accelerated by the proliferation of broadband communications and mobile telephone communications including high quality moving images.
At present, Ethernet (registered trademark) is widely used as a communication protocol which supports the Internet protocol (IP), however, current communication systems have just begun practical communications at 10 Gbps. Imminent requirements exist for the development of communication technologies which provide communication at 40 Gbps or higher for inter-city (metro) communications and the like which involve heavy communication traffic, though not over large distances.
In addition, at present, optical modulation which directly modulates the current of a semiconductor laser (e.g., laser diode (LD)) that is a light source is widely employed in optical transmitters because of its simple configuration. For marine optical cable communications which involve long distance transmissions, external optical modulators are employed because the charping of transmitted light caused by the direct modulation of LD causes degradation in waveforms due to the relationship to group velocity dispersion in optical fibers.
Modulation frequencies beyond 10 GHz cause many optical components to have poor performance. LD-based direct modulation produces a delay in the change in the density of an injected carrier over time, thus encountering difficulties in modulation at a relaxation oscillation frequency or higher. An external optical modulator that uses a ferroelectric crystal experiences difficulties in matching the output impedance of a driving source due to a lower impedance of the device, thereby resulting in a failure in providing sufficient modulation characteristics.
Likewise, in electronic circuits and LSIs for processing digital signals, the transmission of signals at higher speeds results in difficulties in the transmission of signals through metal wires particularly between circuits and between chips, so that there is an immediate need for the development of new technologies for optical fiber lines, optical interconnections, and the like. In this case, since multiple optics are integrated in a chip, another essential requirement, in addition to improved speed, is reduced size.
From the foregoing, it is no exaggeration to say that a key to enabling optical communication and optical wiring technologies at communication speeds exceeding 40 Gbps is to develop small and low-cost optical modulators.
Mach-Zehnder waveguide optical modulator is one of the optical modulators, at present, that have a practical use. This modulator relies on an electro-optical effect to provide a phase difference between waveguided lights which propagate through two arms that are branched in a Y-shape. As the waveguided lights which have propagated through both arms are combined again at a Y-shaped branch, the intensity of the combined light is modulated in accordance with the provided phase difference. This modulator employs a Ti thermally diffused waveguide made of electro-optical crystals of ferroelectric lithium niobate.
An expensive optical modulator for optical communications, in which an intensive optical modulator and an edge emission distributed feedback laser are monolithically integrated, is provided. The intensive optical modulator has advantage of an electro absorption effect in which an inverse bias is applied to an optical semiconductor crystal, a Frantz-Keldish effect, or a quantum confined Shutark effect (QCSE).
On the other hand, up to the present, the optical path length n·d (where n is the refractive index, and d is a geometric thickness) of a Fabry-Perot Ethalon surface resonator was actively changed to control the wavelength sweeping and the transmission wavelength of a filter by repeated multiple reflection interference. If the wavelength of incident light is fixed, this operation corresponds the optical modulation of an optical modulator.
An example of such optics is a variable wavelength filter made of an optical semiconductor material as disclosed in JP-10-136116-A (see pages 3-4 and FIG. 4). This wavelength filter, which includes a multi-quantum well (MQW) layer sandwiched by highly reflective films, varies the wavelength which is transmitted by the filter by applying an electric field between the reflective films to change the refractive index of the MQW layer.
JP-10-136116-A also describes the utilization of a change in the refractive index through a carrier plasma effect and a band filling effect which can be produced through current injection.
In addition, JP-61-088229-A (see pages 2-3 and FIGS. 1-3) discloses another example of a variable wavelength filter made of a transparent dielectric material. This variable wavelength filter is composed of a quartz glass substrate; dielectric multi-layer light reflection films each made of TiO2 and SiO2; a polycrystalline film made of PLZT (La doped oxide zirconium, oxide titanium and oxide lead compound) in a thickness of approximately 200 nm which has an electro-optical effect; and a dielectric multi-layer film identical to the foregoing. These components are sequentially deposited on the quartz glass substrate by a sputtering method to form an Ethalon resonator. The wavelength transmitted by the filter is controlled by applying a voltage between the reflection films, making use of a change in the refractive index which is assumed to be attributable to a secondary electro-optical effect (Kerr effect).
This device encounters difficulties in forming a thick ferroelectric film to increase voltage sensitivity. On the other hand, the ferroelectric film can be deposited by a CVD (chemical vapor deposition) method, vapor deposition, a sol/gel method, and a method of manufacturing a composite structure as disclosed by JP-2002-235181-A (see page 16 and FIGS. 11, 13-14), other than the sputtering method.
However, any of the prior art optical modulators described above cannot successfully meet all requirements for application in optical communication and optical wiring technologies at 40 Gbps or higher such as faster operation, reduction in driving voltage, reduction in size, an arrayed arrangement, lower cost, and the like.
A Mach-Zehnder optical modulator which employs a Ti thermally diffused waveguide made of electro-optical crystals of ferroelectric lithium niobate has dimensions on the order of several centimeters, and a long device length because of a small specific refractive index difference and a small waveguide turnout angle. Also, the Mach-Zehnder optical modulator requires a high driving voltage because of a small electro-optical constant presented by the material. Further, the Mach-Zehnder optical modulator has limitations when operating at 20 GHz because of difficulties in matching a characteristic impedance of a travelling wave electrode, which is applied with a modulation signal, with a driving source.
The lumped circuit type optical modulator that takes advantage of an electro absorption effect with an inverse bias applied to an optical semiconductor crystal, or a quantum confined Shutark effect, entails a high manufacturing cost because of the requirement for advanced and complicated compound semiconductor crystallization technologies and lithography. This makes the lumped circuit type optical modulator unsuitable for applications in optical wiring which often involves a plurality of elements arranged in array.
The optical modulator device (JP-01-136116-A) which includes a wavelength filter made up of a multi-quantum well (MQW) layer of an optical semiconductor sandwiched by highly reflective films and which varies a wavelength transmitted by the filter, entails a high cost because the substrate is limited to crystals which match the condition of crystal growth layer in lattice and there is a need for a crystal growth technology based on advanced apparatuses. Also, difficulties in forming a thick MWQ layer inevitably contribute to a lower sensitivity. Further, since the optical modulator device utilizes the red shift of a fundamental absorption end near the transmission wavelength caused by an applied voltage, a change in the refractive index is accompanied by a coincident change in light absorption, causing the device to invariably suffer from a high light insertion loss. If the blue shift by carrier injection is utilized, noise light, such as naturally emitted light, is generated in addition to a coincidental change in light absorption. Consequently, the technologies described in JP-01-136116-A are not effective for application in light control devices such as an optical modulator, a variable wavelength filter and the like.
In the optical modulator device (JP-61-088229-A) which includes an Ethalon resonator made of PLZT having an electro-optical effect on a quartz glass substrate, it is difficult to form a thick ferroelectric film to increase the sensitivity to the voltage. Supposing that the optical modulator device includes a thick ferroelectric film, the quality of the film would be degraded, resulting in increased light scattering, worsened spatial coherence of light, and increased light loss. Also, since the optical modulator device is estimated to utilize the Kerr effect, the device has a problem of poor controllability due to hysteresis introduced in the voltage versus light transmission characteristic.
The sputtering method used for depositing the ferroelectric material, described in JP-61-088229-A, can produce an optically high quality film in a thickness of 2 μm or less at most. Also, the manufacturing method disclosed in JP-2002-235181 for depositing a ferroelectric film has never successfully produced an optically high quality film. In order to increase the impedance and reduce the voltage of the Ethalon resonator type optical modulator, it is necessary to increase the film thickness five times larger than can be done by conventional methods.